Vaccines activate the body’s natural defenses, thereby reducing the risk of disease. They work by triggering a response from our immune system, which:

• Recognizes the invading microbe (for example, a virus or bacteria;
• Generates antibodies, which are proteins that our immune system produces naturally to fight disease;
• Remembers the disease and how to fight it. If, in the future, we are exposed to the microbe that the vaccine protects against, our immune system will be able to destroy it quickly before we start to feel sick.

Ultimately, vaccines are an ingenious and safe way to induce an immune response without causing disease.

Our immune system is designed to remember. After the administration of one or more doses of a vaccine against a specific disease, we are protected against it, usually for years, decades or even for a lifetime. That’s why vaccines are so effective: instead of treating a disease when it appears, they prevent us from getting sick

Source: WHO